The term “biodiesel” is used to refer to vegetable or animal fat, oil or grease derived diesel, consisting of long-chain alkyl methyl (ethyl or propyl) esters. Biodiesel is distinct from petroleum diesel (also known as petrodiesel or petroleum derived diesel), which is a specific fractional distillate of petroleum fuel oil. Biodiesel is typically produced from a variety of feedstocks, including edible and non-edible fats and oils.
Fats are triglycerides (triesters of glycerol and any of several fatty acids), and may be either solid or liquid at room temperature. Oils are neutral, non-polar molecules with a high hydrocarbon content. Greases are semisolid lubricants which generally consist of a soap emulsified with mineral or vegetable oil. Greases can be of natural origin, and can be either vegetable or animal derived.
Many biodiesel feedstock fats and oils have high purity, and therefore can be easily converted to biodiesel in a predictable way using known processes and known reaction conditions. However, in general pure feedstocks are more costly, the feedstocks having other potential uses.
There is also available a variety of poor quality and less pure sources of fats and oils (and greases), which can potentially be used to produce biodiesel. However, impure feedstocks are generally more problematic, leading to unpredictable reaction conditions and a less pure, less predictable biodiesel product. One such potential feedstock is the fats, oils and greases which are constituents of sewer waste or sewage, in which context they are referred to commonly as “sewer grease”.
Fats, oils and greases gather in sewer waste and are very problematic, leading to the blockage of sewerage systems. Thus they are generally considered a problematic waste product. Nevertheless, such fats, oils and greases do contain hydrocarbons, which are a potential source of valuable products such as, for example, fuels.
Typically, however, sewer grease comprises a number of waste products and aqueous materials. For example, sewer grease contains a large amount of insoluble debris and many different chemicals. Furthermore, sewer grease is very viscous and has a relatively high melting point, which is why it causes sewerage systems to become blocked.
Fuels are required to have a certain degree of purity in order to be usable. For example, biodiesel needs to have a certain purity or FAME (fatty acid methyl ester) value in order to be usable in engines or the like. Current legislation (EN 14214) states that in order to be suitable for use in engines, biodiesel must have a minimum ester content of 96.5%. Furthermore, in order to be usable in modern engines, fuels must have an acceptable cetane number. Cetane number is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of the quality of a diesel fuel. A higher cetane number is ordinarily more desirable as this indicates that a fuel will more easily combust in a diesel engine, giving a smoother (and sometimes more efficient) engine.
In general, the starting material used has a large impact on the quality of the biodiesel obtained, and FAME and the cetane number. In particular, low quality starting materials are normally associated with fuels having low FAME and low cetane number. A number of other measurements determine overall diesel fuel quality including: density, lubricity, cold-flow properties (cold filter plugging point), mono-glyceride content, and sulphur content. These measurements are also typically affected by the quality of the starting material used.
Therefore, due to the impure and unpredictable nature of sewer grease, it has not been practicable to use sewer grease as a source of fats, oils and greases for producing fuels such as biodiesel.
It is an object of the present invention to overcome or mitigate at least some of the problems of the prior art.